1. Field of the Invention
The invention relates to a process for the effective production of highly wear-resistant layers on inductively heatable workpieces. Articles for which their application is possible and useful, are a number of components subjected to abrasive, corrosive, adhesive or sliding wear, preferably made of steel, but also of cast iron or aluminum or titanium alloys. The invention is particularly advantageously applicable to all hardenable steels or steels susceptible to cracking, such as, e.g., heat-treatable steels, tool steels, cold work steels, bearing steels and hardenable sorts of gray iron. Components for which this invention can be used are, e.g., motor components, pump shafts, heat exchangers and heat exchanger pipes, forming tools, components in the oil production industry, camshafts, cam levers, valves and the like.
2. Discussion of Background Information
It is known that components made of metallic construction materials can be more effectively and selectively protected against wear with the aid of laser build-up welding than with the classic processes such as, e.g., TIG or plasma powder build-up welding, flame spraying or plasma spraying. The reasons for the better tribological and corrosive resistance lie, i.a., in the lower dilution with the basic material of the layers that can be produced with the laser. This is ultimately possible due to the very localizable and controllable energy input with comparatively high power densities and the resulting short process times.
For a number of applications, in particular those with large surfaces subject to wear stress, however, there is the negative effect that the cost of the layers produced by laser build-up welding is too high. The reason for this drawback is that the specific costs of providing energy are much higher with a laser than with other conventional energy sources such as, e.g., TIG or plasma torch.
A second drawback is that the layers produced by means of the laser build-up welding, i.a., tend more towards crack formation than those produced by conventional methods, if a change is made to harder and thus less ductile coating materials or martensitic hardenable substrate materials. The reason for this drawback is due to the fact that the very intensive energy input aimed at and realized with the laser action is associated with very high power densities which lead to large temperature gradients and thus to such high transient thermal tensions in the cooling phase that cannot be tolerated by a number of coating and substrate materials without cracking.
To eliminate both drawbacks at the same time, it is known to preheat the areas to be coated with an inductive additional energy source (cf., e.g., B. Brenner, V. Fux, A. Wetzig, S. Nowotny: Induktiv unterstütztes Laserauftragschweiβen—eine Hybridtechnologie überwindet Anwendungsgrenzen, 6th European Conference on Laser Treatment of Materials, Stuttgart Sep. 16-18, 1996, conference materials p. 477-484). The inductive preheating has a particularly effective impact on the reduction of the temperature gradient and the reduction of the transient stresses thus possible, because the energy input occurs not only over the surface but at a depth that can be established by means of the induction frequency. Moreover, the specific energy provision costs for the coupled inductive energy are at least one order of magnitude lower than for laser energy.
Yoshiwara and Kawanami (Method for surface-alloying metal with a high-density energy beam and an alloy steel, EP 0190378A1) thus claim a process with which an inductor or an oxygen acetylene burner permanently connected to a laser beam focusing unit in feed direction before the laser-irradiated area act on the workpiece. The area thus preheated is larger than that subsequently irradiated. Accordingly, an unsteady preheating temperature field results with a maximum that is shifted somewhat towards the laser beam and runs before the temperature field produced by the laser beam. In addition to this, the same arrangement can additionally be arranged after the laser beam point, thus realizing a postheating. The amount of energy that is supplied by the second energy source should be a substantial part of the necessary total process energy, but remain less than the amount of energy provided by the laser beam. The arrangement described in EP 0190378A1 can preferably be used for very large workpieces. With it, using an oxygen acetylene burner (data for an inductive preheating are not given), it was possible to alloy an alloy claimed in the same patent in the surface layer of the substrate material 17Cr2W1Ni (tool steel; chemical composition: 1.74% C; 17.4% Cr; 1.78% W; 0.92% Ni; balance Fe) without cracks at a peak temperature of the preheating cycle of 700° C. The feed rate achieved was 75% higher than the rate achieved without preheating (2.4 m/min with 10 kW laser power).
The drawback of the process is that the feed rate cannot be increased to the extent that should actually be possible from estimates of the energy balance. The reason for this is that the temperature fields that can be produced with an inductor (lower energy transmission effectiveness through use of the inductive outer field) or with an oxygen acetylene burner in the claimed arrangement cannot be adequate adapted to the requirements of laser alloying or laser build-up welding. If the additional energy source that yields approximately the same energy as the laser beam, but features a much lower power density, moves in advance of and at the same feed rate as the laser beam, an optimum maximum temperature cannot be achieved. Moreover, the temperature of the preheating temperature field at the time the laser beam passes has dropped so much that no substantial effect can result regarding an increase of the feed rate.
Higher feed rates are achieved according to the solution from EP 0190378A1 by subjecting the entire component to an additional intensive preheating in a furnace (inventive method (2)) before the treatment described above. The preheating temperature of the furnace heating is up to 600° C. If the peak temperature of the short-time preheating cycle is increased to 800° C. by means of the above-mentioned oxygen acetylene burner, the feed rate can be increased to 5.4 m/min with the same laser parameters or accordingly to 225%.
However, it has proven to be a drawback that the pretreatment in the furnace is very complicated, lengthy and expensive. Moreover, it has a detrimental effect that the components have to be transported, positioned and clamped while in a hot condition. The reason for both drawbacks is that the parts have to be thoroughly heated in a separate device.
A further drawback is that, due to the increasing oxidation of the component, the preheating temperature is limited to approximately 600° C. The possibilities of a further increase in speed have thus also been exhausted.
A higher preheating temperature affects the quality of the alloyed layer in that oxide inclusions or pores occur which reduce the mechanical load capacity of the layers and their wear resistance. In the inventive step mentioned an attempt is made to solve this problem by adding deoxidizing elements and slag-forming agents to the coating material. However, due to stochastic melting bath turbulences, it cannot be ensured that all slag particles or metal oxides rise to the melting bath surface. These coats are therefore not suitable for stresses in which very high surface pressings or cyclical tensions occur.
Guilloud, Dekumbis and Gonseth (EP 0462047B1 “Process and apparatus for the formation of surface layers on articles and articles with a surface layer formed according to this process”) disclose a process for laser build-up welding of the sealing surfaces of engine valves with which before the laser build-up welding at least the entire functional surface of the component to be coated is uniformly heated by an inductive preheating to a constant temperature, the laser beam is brought into the center or near the center of the inductively heated area and the inductive heating of the entire preheated area is maintained during the laser build-up welding. A temperature of 800° C. is given as the inductive preheating temperature for the claimed exemplary use of automobile valve casings. In a further work, Dekumbis (“Beschichten von Automobilventilen mit der Laserinduktions-Technologie” elektrowärme international (Vol. 51) 1993 B3, September issue, p. B113-B115) gives the increase in the process velocity thus achieved at only 30%, but without specifying absolute values. This indicates that far lower preheating temperatures were used in practice.
A considerable drawback of the process is that evidently only very slight increases in the process velocity could be achieved. The most important reason for this is that the entire functional surface has to be kept at the very high preheating temperature for the full duration of the laser build-up welding.
An increase in the preheating temperature to higher temperature values for greater critical ranges, such as are necessary for conducting this process on larger components, is not possible due to the oxide and scale formation that greatly increases with the temperature. This applies to a greater extent when normal steels are used, which are much less oxidation- and scale-resistant than the valve steels considered, the chemical composition of which has been optimized for a use at higher temperatures.
Another drawback of the process is that it is difficult to apply to larger and in particular intricately formed workpieces. The reason for this is that very intricately formed inductors have to be developed, tested and optimized.